oid2name has done little effort to keep an interface consistent with
other binary utilities:
- -H was used instead of -h/-host.  This option is now marked as
deprecated, still its output is accepted to be backward-compatible.
- -P has been removed from the code, and was still documented.
- All options gain long aliases, making connection options more similar
to other binaries.
- Document environment variables which could be used: PGHOST, PGPORT and
PGUSER.

A basic set of TAP tests is added on the way, and documentation is
cleaned up to be more consistent with other things.

Author: Tatsuro Yamada
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/c7e7f25c-1747-cd0f-9335-390bc97b2db5@lab.ntt.co.jp